Question

The energy stored in the capacitor as shown in Fig. (a) is $4.5 \times 10^{-6} \,J$. If the battery is replaced by another capacitor of $900 \,pF$ as shown in Fig. (b), then the total energy of system is

• A. $4.5 \times 10^{-6}\, J$
• B. $2.25 \times 10^{-6}\, J$
• C. zero
• D. $9 \times 10^{-6} \,J$

Answer 1

Answer: (B)

Energy stored in the capacitor in Fig (a)
$\frac{1}{2} \frac{Q^{2}}{C}=4.5 \times 10^{-6} J$
If battery in Fig. (a) is replaced by capacitor in Fig. (b),
total energy stored
$=\frac{1}{2}\left(\frac{1}{2} \frac{Q^{2}}{C}\right)$
$= \frac{1}{2} \times 4.5 \times 10^{-6}$
$= 2.25 \times 10^{-6}\, J$